Their lawyer Nick Turner, of Russell and Co, said the case could pave the way for other children who were unborn when their parents were killed to bring compensation claims.

Mr Turner will argue that the children have been deprived of Mr Thomson's income. He was a probationer dairyman when he died.

The 22-year-old female driver has been arrested on suspicion of causing death by dangerous driving and is now on bail.

The two police officers, who were pursuing the car on an unlit lane in dense fog in an unmarked BMW X5, have been put under investigation.

Mr Turner said the case could involve a compensation clam for up to half a million pounds.

He said: "I've been doing civil actions against the police for 25 years and I can't recall ever coming across a case where the plaintiff is a three-month-old child born a week after the police killed her father," he said.

The sisters' claim will be brought under the Fatal Accidents Act 1976, which allows relatives of those killed by the wrongdoing of others to claim compensation. This case will clarify whether the act covers unborn children.

An IPCC spokesman said: "Our independent investigation into a fatal road traffic incident in the area of Pontrilas, near Hereford in which Liam Thomson died, is ongoing.

"We have served notices on two West Mercia Police officers to advise them they are subject to investigation."